This invention relates to metal and ceramic containing parts that are formed from powder, and more particularly to a method of making and control of the dimensions of such parts.
Standard practice in powder metallurgy falls into two basic categories. In powder pressing, powder is placed between two hardened steel dies and compacted, typically to densities of 80% or more. This compaction involves the deforming the powder particles so that they mechanically interlock, thus creating a porous skeleton. Often this skeleton is subsequently infiltrated with a lower melting point material in order to form a fully dense part. For example, skeletons of steel powder are often infiltrated with copper alloys.
Alternatively, the techniques of metal injection molding are used, where a powder is mixed with a binder material and is injected into a die. After the binder has solidified, the powder component is removed. This green part is typically approximately 60% dense. The binder is then burned off or removed chemically and the skeleton is then sintered. Generally speaking, the skeleton is sintered to near full density.
In the fabrication of metal components by powder metallurgy, the control of the dimensions of the final component is often an important issue. Such dimensional control becomes an especially important issue when the components being made are to be used as tools and dies for the fabrication of other components by forming processes.
In a known tooling process, described in general in U.S. Pat. No. 4,554,218 entitled INFILTRATED POWDERED METAL COMPOSITE ARTICLE, issued on Nov. 19, 1985, in the name of Gardner, et al., a skeleton is formed by packing powder around a form and holding it together with a polymeric binder. After removal from the form, the polymeric binder is burned off and the skeleton is lightly sintered. A subsequent infiltration with a low melting point alloy provides a fully dense part.
The two most common approaches for densifying a powder skeleton are either to sinter it to full density or to fill the voids in the skeleton with a second material. These voids may be filled by infiltration of a lower melting point metal, or by infiltration of a polymeric material such as an epoxy.
The skeleton that is formed in the first step of the process may range in density from 55-85%. If sintering to full density is chosen, a significant amount of additional shrinkage must be incurred. The shrinkage arises because material migrates from within the bodies of particles to form larger necks between particles. For example, if a skeleton of 60% density is sintered to full density, the shrinkage must be approximately 18% linear. This large amount of shrinkage can cause significant problems if the goal is to maintain good dimensional accuracy. For example, if a 1% variation in shrinkage is encountered then a dimension which requires a 15% shrink will have an uncertainty of 0.15% of original. Thus, a 10 cm dimension will be uncertain by 0.15 mm, a very significant error when precision components are considered. For this reason, the method of creating a skeleton and then sintering the skeleton to full density in a secondary operation is not attractive when precision parts are concerned.
A processing technique that uses powders has become known as xe2x80x9cthree-dimensional printingxe2x80x9d (xe2x80x9c3D Printingxe2x80x9d) and is described in general in numerous patents, including: U.S. Pat. No. 5,204,055, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,340,656, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,387,380, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Cima, Sachs, Fan, Bredt, Michaels, Khanuja, Lauder, Lee, Brancazio, Curodeau, and Tuerck; U.S. Pat. No. 5,490,882, entitled PROCESS FOR REMOVING LOOSE POWDER PARTICLES FROM INTERIOR PASSAGES OF A BODY, by Sachs, Cima, Bredt, and Khanuja; and U.S. Pat. No. 5,660,621, entitled BINDER COMPOSITION FOR USE IN THREE-DIMENSIONAL PRINTING, by James Bredt; U.S. Pat. No. 5,771,402, issued Jul. 7, 1998, entitled ENHANCEMENT OF THERMAL PROPERTIES OF TOOLING MADE BY SOLID FREE FORM FABRICATION TECHNIQUES, by Allen, Michaels, and Sachs; and U.S. Pat. No. 5,807,437, issued on Sep. 15, 1998, entitled HIGH SPEED, HIGH QUALITY THREE DIMENSIONAL PRINTING, by Sachs, Curodeau, Fan, Bredt, Cima, and Brancazio. All of the foregoing 3D Printing patents are incorporated herein fully by reference.
3D Printing is also disclosed and discussed in co-pending, co-assigned applications, including: U.S. Ser. No. 08/600,215, filed Feb. 12, 1996, entitled CERAMIC MOLD FINISHING TECHNIQUES FOR REMOVING POWDER, by Sachs, Cima, Bredt, Khanuja, and Yu; U.S. Ser. No. 08/856,515, filed May 15, 1997, entitled CONTINUOUS INK-JET DROPLET GENERATOR, by Sachs and Serdy; U.S. Ser. No. 08/831,636, filed Apr. 9, 1997, entitled THREE DIMENSIONAL PRODUCT MANUFACTURE USING MASKS, by Sachs and Cima; U.S. Ser. No. 60/060,090, filed Sep. 26, 1997, entitled REACTIVE BINDERS FOR METAL PARTS PRODUCED BY THREE DIMENSIONAL PRINTING, by Sachs, Yoo, Allen, and Cima (provisional application); U.S. Ser. No. 60/094,288, filed Jul. 27, 1998, entitled METHOD OF MAKING INJECTION MOLDS HAVING COOLING CHANNELS THAT ARE CONFORMAL TO THE BODY CAVITY, by Xu and Sachs (provisional application); and PCT application PCT/US98/12280, filed Jun. 12, 1998, which designates the U.S., entitled JETTING LAYERS OF POWDER AND THE FORMATION OF FINE POWDER BEDS THEREBY, by Sachs, Caradonna, Serdy, Grau, Cima, and Saxton. All of the foregoing 3D Printing patent applications (and provisional application) are incorporated herein fully by reference.
The flexibility of the 3D Printing process makes it possible to construct a part out of any material available in powdered form. Such a part can possess almost any geometry, including overhangs, undercuts, and internal volumes. The 3D Printing process was initially developed for the production of ceramic shells and is also useful in making metal parts. One key use for such a system is the production of injection molding tooling for plastic parts. Injection molds are used to make a vast array of items, ranging from toys to floppy disks. The lead times for the production of such tools generally range from a few weeks to several months. The rapid production available via the 3D Printing process can greatly reduce this lead time, thereby alleviating a bottleneck and reducing the duration of product development.
Designers of plastic parts often call for fairly tight part tolerances. Therefore, the tolerances of the injection molds are also critical. In a known 3D Printing process, metal parts are produced by printing a polymer binder into stainless steel powder. The bound parts are subsequently furnace-treated to lightly sinter and debind. This debinding step requires burning out the polymer binder, which is typically a messy process that raises maintenance challenges. Once lightly sintered and debound, the parts are then infiltrated with a molten metal alloy. Sometimes, in order to prevent gravitational slumping and other forms of part distortion, the green part is loosely repacked in refractory material to support unsupported sections. This repacking is called xe2x80x9csetteringxe2x80x9d. In some cases, the infiltration step is accompanied by a second, more severe sintering step.
Such a more severe sintering step is provided to achieve certain mechanical properties, such as higher impact toughness, yield and tensile strengths. It is believed that these properties improve due to increased necking between particles. However, it is just this necking during sintering that causes shrinkage.
These post-printing processes cause a total linear dimensional change of approximately xe2x88x921.5%xc2x10.2%. In other words, the average shrinkage is 1.5%. However, there is an uncertainty in the value of shrinkage of xc2x10.2%. For large parts, these uncertainties become extremely significant (in the absolute). This uncertainty results in the loss of some dimensional control of the parts. Furthermore, growth of the part is often observed during infiltration. While this growth can be predicted to a certain extent, this growth also carries an uncertainty, which then adds to the uncertainty of dimensions of the final infiltrated part.
The known 3D Printing process can compensate for a given amount of predicted shrinkage by beginning with a larger green part. The shrinkage that occurs during post-processing is accounted for when the green part is printed. However, as mentioned, the shrinkage has a certain amount (xc2x10.2%) of error associated with it and furthermore is not uniform. This can lead to warped parts whose final dimensions bear some uncertainty. For this reason, alternate powder/binder systems for metal parts are desired.
Thus, there is a need to improve the dimensional control of metal parts produced by 3D Printing. One approach would be to use materials systems that have a lower average shrinkage. It is further presumed that the uncertainty (error) in shrinkage would be reduced commensurately. For example, a system with an average shrinkage of 0.1% will certainly have a variation (error) in shrinkage of less than xc2x10.1%. Further, a similar ratio of uncertainty in shrinkage to average shrinkage as is found today may be found, resulting in a variation in shrinkage of less than 0.02%. For example, if a tool with a dimension of 10 inches (25.4 cm) is fabricated by known 3D Printing techniques, the tolerance that can be held is +/xe2x88x920.02 inches (0.05 cm). However, with a low shrink material system that has an uncertainty of +/xe2x88x920.02%, the tolerance could be as low as +/xe2x88x920.002 inches (0.005 cm).
It is not known in the art how to achieve a lower average shrinkage. A major contributor to the shrinkage is the sintering step, which draws major bodies of the powder granules together toward each other. Thus, a goal of the present invention is to minimize or eliminate sintering of the powder granules, and to the extent that any sintering arises, to minimize any shrinkage associated therewith. Because sintering would be reduced or eliminated according to such a possible approach, if densification is desired, it would be necessary to bring the part to full density by some other means, such as infiltration.
The chosen method of reducing shrinkage and distortion must also provide sufficient part strength and edge definition, and still permit successful infiltration of an infiltrant, if densification is desired. Furthermore, the chosen method must not adversely affect the mechanical properties of the finished part, such as yield and tensile strengths, impact toughness, etc. In fact, it would be preferred that the method of forming the skeleton positively aid in attaining desirable mechanical properties.
An additional aspect of known sintering techniques that is undesirable in some circumstances is the need to setter, i.e. to support a green part in refractory material to eliminate slumping and other deformation during sintering. This settering step is required because the material of the initial powder bed is essentially identical to that which will form the final part. Thus, if the entire bed were to be heated to a temperature that would sinter the locations of the powder that will make up the part, the entire bed would sinter into a monolithic block. The present solution to this problem, settering, has undesirable aspects. Settering requires an extra step of removing the unprinted regions of the original powder bed; packing the fragile green part in another powder bed, and insuring that all of the fragile regions of the part are supported; and again removing powder from the part after the sintering step. Further, there is an inherent conflict in sintering a part that is settered. The sintering part tends to shrink. However, the supporting settering powder tends to resist any such shrinkage. Moreover, settering is only an adjunct to a sintering step, which has its own drawbacks, as already mentioned.
There are also known techniques of mixing a binder solution with powder particles, to achieve a homogeneous mixture that is then molded, allowed to dry in a bound state, and then heat treated. The particles and the liquid must be thoroughly mixed, with much relative motion between the particles during mixing, to avoid clumping and achieve a homogeneous mixture. A drawback of mixing the binder to achieve a homogeneous mixture is that large amounts of liquid must be used to achieve a slurry that is sufficiently flowable to assume the desired shape. Consequently, the packing fraction of the powder is lower, and thus the green part is less dense. The relatively low packing fraction of the powder gives rise to material inhomogeneities, as the slurry flows to fill the mold. A further drawback of this method of providing the binder liquid is that it is not possible, or at best, very difficult, to provide the binder liquid in preselected regions within the body. In general, the entire body will be bound.
Thus, an objective of the present invention is to provide a method for fabricating a skeleton with little or no average shrinkage required for its formation and with a higher degree of certainty as to the variation in the shrinkage. It is a further objective of the invention to provide a skeleton that is highly resistant to distortion during subsequent densification steps.
Another objective of the invention is to provide a liquid binder at specific locations in a powder body, allowing the particles to come to rest substantially in the same positions that they will occupy in the final body, without requiring significant relative motion of the powder particles, and also obtaining a homogeneous combination of powder and liquid, with a high packing fraction.
It is a further objective of the invention to provide a preform that is ready for infiltration, without requiring that the preform be first treated in a first powder bed, and then removed and repacked in a second powder bed, from which it is also subsequently removed.
Still another object of the invention is to be able to retain fine features, as printed, by safely supporting these printed features through all processing steps.
Yet another object of the invention is to avoid sintering the main constitutive powder granules throughout the body of the part, with its inherent conflict between the shrinkage of the sintering part and the resistance to shrinkage of any support medium.
It is another object of the invention to provide necking in a body of particles, with corresponding improvement in desirable mechanical properties, but without the shrinkage that arises in connection with necking that occurs during sintering.
It has been determined that if a process is used where the powder granules do not themselves sinter, shrinkage and the variation, or error in shrinkage, can be greatly reduced. Further, if a step that is different from sintering is used to give strength to the forming shape at a temperature that is below the sintering temperature of the main powder body, the part can be shaped and strengthened at this lower temperature in the original powder bed, without the need for settering. Thus, the delicate portions of the part are adequately supported.
The current invention creates the powder geometry in two distinct steps. In a first step, a powder is placed so that the particles are in maximum contact with each other. Further, the body of powder is shaped as desired, for instance in a layer, or in or around a mold. In a second step, a further material is added in a liquid carrier. The further material will provide for the bonding between powder particles. In contrast to a conventional method, where a fugitive binder is removed and the powder particles themselves sinter together and thus provide the material that join them, in the present invention, the material that joins the powder particles is provided as an independent material. It is a special aspect of the current invention that two different materials are required. One as the main body material and the second as the source for the binding material. Further, there is no movement of the powder particles after they have been placed, i.e., there is no mechanical mixing.
A preferred implementation is to provide a layer of powder and then to provide the further material within that layer and to repeat this process layer after layer according to a three dimensional printing method, such as is described in U.S. Pat. No. 5,204,055, mentioned above. According to another preferred implementation, rather than using 3D Printing, the liquid carrier is provided to powder packed in or around a mold.
According to one aspect of the invention, a solution of metal salt (for instance, an aqueous solution) is printed into a powder bed at locations where it is desired to bind the powder granules together. The salt solution accumulates at necks between the granules, due to the operation of surface tension. A constituent that originates from the salt solution forms at the necks and mechanically joins the granules together. The constituent may be crystallized salt itself, or it may be a metal that results from reduction of the salt or some species in between the transformation of salt into metal.
For instance, silver nitrate (AgNO3) solution printed onto stainless steel powder precipitates at necks between granules as the solution dries. Another suitable salt is a Rochelle salt C4H4KNaO6.4H2O (sodium potassium tartrate).
In some cases (e.g. the rochelle salt and epsom salts), the precipitated crystallized salt alone is strong enough to maintain the shape of the part as it is subsequently infiltrated.
In other cases (e.g. silver nitrate), the crystallized salt is subsequently reduced by heating it in a suitable environment, which reduction causes the metal (e.g. silver) to deposit onto the powder. The conditions of reduction can be chosen so that the metal deposits onto the powder in a thin film. Regarding the reducing atmosphere, some salts require the presence of hydrogen or some other reducing gas in the environment. However, some salts require only that they be heated up and the compound itself disassociates into a metal and other byproducts. For instance, silver nitrate melts at 212xc2x0 C. forming a yellowish liquid. At 440xc2x0 C. it decomposes into silver, nitrogen, oxygen and nitrogen oxides.
Alternatively, rather than first drying the solution, the salt and a metal powder can be chosen such that upon contact with the metal powder, the salt in the solution reduces to metal, in the solution. A principal example of this embodiment of the method of the invention is where the salt and a portion of the metal powder undergo an electrochemical xe2x80x9cdisplacementxe2x80x9d reaction. As a result of this reaction the metal of the salt (or a portion of it) is deposited onto the metal powder.
This deposition of the metal from ionic solutions consists of adding electrons to the dissolved metal ions (of the salt). These electrons are provided by the metal powder as it partially dissolves into the salt solution. A specific example is when silver is the metal of the salt, e.g., silver nitrate, and copper is the metal of the powder. Another example is silver carbonate dissolved in ammoniated water undergoing a displacement reaction against molybdenum powder.
With both the reduction and the displacement reaction embodiments, in some cases, the body, after reduction of the metal, is not strong enough for its intended purpose, or for subsequent treatment. In some such cases, it is possible to heat the body further, to cause either sintering or melting of the metal that has come from the salt. This approach is possible when the powder in which the salt has been printed has a high enough sintering temperature, TMAX, so that it will not begin to sinter or melt during the subsequent heat treatment.
In cases where the powder into which the salt solution has been printed does not have a high enough sintering temperature, the powder may be held together tightly enough by either the salt crystals, or reduced metal so that it can be removed from the original powder, settered in another, higher temperature powder, and then fired to either reduce the salt to metal, if reduction has not yet taken place, or to sinter or melt the metal from the salt to provide additional strength. It is important to note that only the small amounts of deposited metal are sintering or melting in this case, and not the primary metal powder of the body. Thus, many of the problems associated with conventional sintering are not present. In some cases, the crystallized salt form is not strong enough to remove from the surrounding powder but it is desired to remove the body from the powder at that crystallized stage. In those cases, it is also possible to add a polymeric binder to the salt solution that is printed. This polymer binder binds the printed part loosely, so that it may be removed and settered, and further heat treated, as discussed above.
As such, a preferred embodiment of the invention is a method for forming a body from powder using a three-dimensional printing process. The method includes the steps of: providing a layer of powder of a first material, granules of the powder layer contacting one another and printing on the layer of powder, a liquid vehicle that contains a salt that will cause the powder granules to be bound together. The steps of providing a layer of powder and printing a liquid on the layer are repeated additional times until a desired amount of printed powder has been provided. The printed liquid and powder are maintained under conditions such that a further material, which is different from the first material, and which originates from the salt solution, forms at interfaces between the powder granules, and binds adjacent granules to each other within each layer and between layers. The further material may be crystallized salt, or reduced metal. The maintaining step typically includes drying, and heat treating. The drying can occur in between successive steps of powder deposition, or after all or a larger part of the body to be formed has been formed.
According to a preferred embodiment, necks of the further material arise at the points of contact between the granules. The subsequent processing may included heat treating these necks to form metal films, or strong metal bodies.
According to another embodiment of the invention, again in a three dimensional printing process, the powder used is a metal powder, and the liquid vehicle contains a salt that is related to the metal of the powder such that upon contact, metal is deposited onto the powder, even while the liquid is present. This is typically by what is known as a displacement reaction.
Still another preferred embodiment of the invention is a method for forming a body from powder, comprising the steps of: providing a volume of powder of a first material, granules of the powder volume contacting one another. While maintaining the granules of the volume of powder substantially stationary relative to each other, a liquid vehicle that contains a salt, which will cause the granules to be bound together is provided in the volume of powder. The liquid and powder are maintained under conditions such that a further material, which is different from the first material and which originates from the salt solution, forms at interfaces between the powder granules, and binds adjacent granules to each other. This embodiment is not limited to a layered method, and can be used for molding.
The variations discussed above with respect to the three dimensional printing embodiments also are important for this more general embodiment. The further material can be crystallized salt, reduced metal, and the metal can be reduced according to a displacement reaction. Further processing can include removing the formed part from the original bed when bound by either crystallized salt, reduced metal or a binder, all of which arise at necks between the particles. Further processing can include elevated temperature treatment to sinter or melt the deposited salt or metal, along with settering, if necessary.
According to yet another preferred embodiment , the invention is a method for forming a body from powder with virtually no shrinkage during processing, comprising the steps of: providing a volume of powder of a first material, granules of the powder volume contacting one another; providing in the volume of powder, a liquid vehicle that contains a salt, which will cause the granules to be bound together; and maintaining the liquid and powder under conditions such that a further material, which is different from the first material and which originates from the salt solution, forms at interfaces between the powder granules, and binds adjacent granules to each other, and strengthens the body while shrinking no more than 0.5% linear.
According to still another preferred embodiment, the invention is a Solid Freeform Fabrication process, where powder and salt, are co-provided, which salt causes the powder particles to be bound together.
Yet another preferred embodiment of the invention is a method for forming a body from powder, comprising the steps of providing a volume of powder of a first material, granules of the powder volume contacting one another. A quantity of a salt is provided in the volume of powder, which salt will cause the granules to be bound together. The salt and powder are maintained under conditions such that a further material, which originates from the salt, adheres to said powder granules at interfaces between the powder granules.
Still another preferred embodiment of the invention is a laser method for forming a body from powder, comprising the steps of: providing a volume of powder of a first material, granules of the powder volume contacting one another. In the volume of powder, a quantity of a salt, is provided, which salt will cause the granules to be bound together. Laser energy is applied to the salt and powder under conditions such that a further material, which originates from the salt, adheres to the powder granules at interfaces between the powder granules.
Another preferred embodiment of the invention is a fused deposition method for forming a body from powder, comprising the steps of: providing a volume of powder of a first material, granules of the powder volume contacting one another; and providing a binder material, that includes in it a dissolved salt, which salt will cause the granules to be bound together. Using a fused deposition modeling step, the salt containing binder material is mixed with the volume of powder, and then extruded under conditions such that a further material, which originates from the salt, adheres to said powder granules at interfaces between the powder granules.